Integrated NOx and PM reduction devices for the treatment of emissions from internal combustion engines

ABSTRACT

One concept of the inventors relates to a system and method in which a particulate filter comprises at least about 40% by weight of an NOx adsorbant. The filter can be used as both an NOx trap and a particulate filter. By constructing the filter elements using a substantial amount of NOx adsorbant, a large volume of NOx adsorbant can be incorporated into the particulate filter, which substantially reduces the volume and expense of an exhaust system that includes both a catalytic diesel particulate filter and an NOx trap having a large quantity of NOx adsorbant. In a preferred embodiment, the filter also oxidizes NO to NO 2 . In another preferred embodiment, an SCR catalyst is position downstream of the filter elements.

FIELD OF THE INVENTION

The present invention relates to the field of pollution control devicesfor internal combustion engines, especially diesel engines and lean burngasoline engines.

BACKGROUND OF THE INVENTION

NO_(x) emissions from vehicles with internal combustion engines are anenvironmental problem recognized worldwide. Several countries, includingthe United States, have long had regulations pending that will limitNO_(x) emissions from vehicles. Manufacturers and researchers have putconsiderable effort toward meeting those regulations. In conventionalgasoline powered vehicles that use stoichiometric fuel-air mixtures,three-way catalysts have been shown to control NO_(x) emissions. Indiesel powered vehicles and vehicles with lean-burn gasoline engines,however, the exhaust is too oxygen-rich for three-way catalysts to beeffective.

Several solutions have been proposed for controlling NOx emissions fromdiesel powered vehicles and lean-burn gasoline engines. One set ofapproaches focuses on the engine. Techniques such as exhaust gasrecirculation and homogenizing fuel-air mixtures can reduce NOxemissions. These techniques alone, however, will not solve the problem.Another set of approaches remove NOx from the vehicle exhaust. Theseinclude the use of lean-burn NO_(x) catalysts, lean NO_(x) traps (LNTs),and selective catalytic reduction (SCR).

Lean-burn NOx catalysts promote the reduction of NO_(x) underoxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere isdifficult. It has proved challenging to find a lean-burn NO_(x) catalystthat has the required activity, durability, and operating temperaturerange. Lean-burn NO_(x) catalysts also tend to be hydrothermallyunstable. A noticeable loss of activity occurs after relatively littleuse. Lean burn NOx catalysts typically employ a zeolite wash coat, whichis thought to provide a reducing microenvironment. The introduction of areductant, such as diesel fuel, into the exhaust is generally requiredand introduces a fuel economy penalty of 3% or more. Currently, peak NOxconversion efficiency with lean-burn catalysts is unacceptably low.

A lean NOx trap (LNT) is an NOx adsorber combined with a catalyst forNOx reduction. The adsorber removes NOx from lean exhaust. Periodically,the adsorber is regenerated by creating a reducing environment. In thereducing environment, NOx is reduced over the catalyst. The adsorbant isgenerally an alkaline earth oxide adsorbant, such as BaCO₃ and thecatalyst can be a precious metal, such as Ru.

SCR involves the reduction of NOx by ammonia. The reaction takes placeeven in an oxidizing environment. The NOx can be temporarily stored inan adsorbant or ammonia can be fed continuously into the exhaust. SCRcan achieve NOx reductions in excess of 90%, however, there is concernover the lack of infrastructure for distributing ammonia or a suitableprecursor. SCR also raises concerns relating to the possible release ofammonia into the environment.

U.S. Pat. No. 6,560,958 describes an LNT system in which hydrogen-richsynthesis gas (syn gas), including H₂ and CO, is used as a reductant toregenerate the adsorbant. The syn gas is produced from diesel fuel in aplasma converter. Periodically, the LNT is taken offline from theexhaust system and supplied with the syn gas. A dual adsorber system isalso described.

U.S. Pat. No. 6,732,507 describes a hybrid exhaust treatment systemusing an LNT and an SCR reactor in series. The SCR reactor capturesammonia produced by the LNT during regeneration and uses the capturedammonia to increase the extent of NOx conversion.

U.S. Patent Application Publication No. 2004/0052699 describes anexhaust treatment device in which the functionalities of a catalyticparticulate filter and a NOx adsorber-catalyst are combined into asingle device. In one embodiment, a wash coat comprising an NOxadsorbant is applied to a surface of a filter element.

There continues to be a long felt need for reliable, affordable, andeffective systems for removing NOx and particulate matter from theexhaust of diesel and lean-burn gasoline engines.

SUMMARY OF THE INVENTION

One concept of the inventors relates to a system and method in which aparticulate filter comprises at least about 40% by weight of an NOxadsorbant. The filter can be used as both an NOx trap and a particulatefilter. By constructing the filter elements using a substantial amountof NOx adsorbant, a large volume of NOx adsorbant can be incorporatedinto the particulate filter, which substantially reduces the volume andexpense of an exhaust system that includes both a catalytic dieselparticulate filter and an NOx trap having a large quantity of NOxadsorbant. In a preferred embodiment, the filter also oxidizes NO toNO₂. In another preferred embodiment, an SCR catalyst is positiondownstream of the filter elements.

The forgoing summary encompasses certain of the inventors' concepts. Itsprimary purpose is to present these concepts in a simplified form as aprelude to the more detailed description that follows. The summary isnot a comprehensive description of what the inventors have invented.Other concepts of the inventors will become apparent to one of ordinaryskill in the art from the following detailed description and annexeddrawings. Moreover, the detailed description and annexed drawings drawattention to only certain of the inventors' concepts and set forth onlycertain examples and implementations of what the inventors haveinvented. Other concepts of the inventors and other examples andimplementations of their concepts will become apparent to one ofordinary skill in the art from that which is described and/orillustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a particulate filter incorporatingan SCR catalyst.

FIG. 2 is a schematic illustration of a particulate filter incorporatingan SCR catalyst in a different way.

FIG. 3 is a schematic illustration of a power generation system.

FIG. 4 is a schematic illustration of another power generation system.

FIG. 5 is a schematic illustration of an exhaust treatment system.

FIG. 6 is a schematic illustration of a particulate filter incorporatingan SCR catalyst and an NOx adsorbant.

FIG. 7 is a schematic illustration of another particulate filterincorporating an NOx adsorbant.

FIG. 8 is a schematic illustration of another particulate filterincorporating an NOx adsorbant and an SCR catalyst.

DETAILED DESCRIPTION OF THE INVENTION

A particulate filter is a relatively large device and is only one ofseveral devices that may be required in a diesel exhaust system to meetemissions control regulations. Incidental to its main function, which isto physically screen particulate matter from exhaust gases, a typicalparticulate filter occupies a large volume and presents a large contactarea to the exhaust gases. The space taken up by a particulate filterand/or the filter's large contact area can be used to facilitate asecond function. In one aspect of the invention, that function is thatof an SCR catalyst bed.

FIG. 1 is a schematic illustration of an exemplary particulatefilter/SCR catalyst 10. The device 10 comprises filter elements 11 andcatalyst elements 12. The filter elements 11 are porous and thestructure of the device 10 generally causes exhaust gases to passthrough the filter elements 11. The catalyst elements 12 comprise anammonia SCR catalyst. In FIG. 1, the ammonia SCR catalyst is formed intoa porous wash coat that lies over the external surfaces of the filterelements 11. Optionally the SCR wash coat covers only the inlet side ofthe filter elements 11 and optionally the SCR wash coat covers only theoutlet side of the filter elements 11.

FIG. 2 illustrates another particulate filter/SCR catalyst 14. In thisembodiment, the ammonia SCR catalyst forms a wash coat that conforms tothe high internal surface area of filter elements 15, whereby the SCRcatalyst is disposed within the filter elements 15. The high internalsurface area of the filter elements 15 and the flow of exhaust gasesthrough those filter elements provides a high degree of contactingbetween the exhaust gases and the catalyst, thereby making efficient useof the catalyst and avoiding the need for a separate ammonia SCRcatalyst device where an ammonia SCR catalyst is desired.

A particulate filter/SCR catalyst can have any of the configurationssuitable for a diesel particulate filter. Examples of suitableconfigurations include monolithic wall flow filters, which are typicallymade from ceramics, especially cordierite or SiC, blocks of ceramicfoams, monolith-like structures of porous sintered metals ormetal-foams, and wound, knit, or braided structures of temperatureresistant fibers, such as ceramic or metallic fibers. Typical pore sizesfor the filter elements are about 10 μm or less, although larger poresmay be initially formed in anticipation of pore sizes being reduced bythe application of a catalyst-containing wash coat. On the other hand,the ammonia SCR catalyst can be incorporated into the filter material.

An ammonia SCR catalyst is one that effectively catalyzes a reactionsuch as:4NO+4NH₃+O₂

4N₂+6H₂Oin lean exhaust. Catalysts for this reaction will also reduce otherspecies of NOx. NO_(x) includes, without limitation, NO, NO₂, N₂O, andN₂O₂. Examples of SCR catalysts include oxides of metals such as Cu, Zn,V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Rd, Mo, and W. Other examplesof ammonia SCR catalyst include zeolites, such as ZSM-5 or ZSM-11substituted with metal ions such as cations of Cu, Co, Ag, Zn, or Pt,and activated carbon. A preferred catalyst is a combination of TiO₂,with one or more of WO₃, V₂O₅, and MoO₃, for example about 70 to about95% by weight TiO₂, about 5 to about 20% by weight WO₃ and/or MoO₃, and0 to about 5% by weight V₂O₃. Catalysts of this type are commerciallyavailable and can be tailored by the manufacturer for specificapplications. The typical temperature range in which these catalysts areeffective is from about 230 to about 500° C. If the temperature is toohigh, the ammonia decomposes before reducing NOx.

FIG. 3 is an exemplary power generation system 20 employing aparticulate filter/SCR catalyst 24, which can have the structure ofeither the particulate filter/SCR catalyst 10 or the particulatefilter/SCR catalyst 14. The power generation system 20 comprises aninternal combustion engine 21, which is typically a compression ignitiondiesel engine, an ammonia supply 22, and an optional oxidation catalyst23. The ammonia supply 22 provides ammonia for the NOx reductionreaction in the device 10. The optional oxidation catalyst 23 convertsNO to NO₂ to facilitate continuous removal of accumulated soot from thedevice 10. Converting NO to NO₂ also facilitates the reduction of NO_(x)by NH₃ over the SCR catalyst.

Any suitable method can be used to remove accumulated soot from theparticulate filter/SCR catalyst 24. Two general approaches arecontinuous and intermittent regeneration. An example of continuousregeneration depends on the reaction of soot with NO₂. Soot will reactwith NO₂ at a lower temperature than with O₂. The optional oxidationcatalyst 23 can comprise a transition metal, preferably platinum, andcatalyzes a reaction of NO with O₂ to form NO₂. The combined filter/SCRcatalyst 24 can contain a catalyst to further lower the effectivetemperature for soot oxidation. Examples of catalysts for the oxidationof soot by NO₂ include oxides of Ce, Zr, La, Y, and Nd. A soot oxidationcatalyst is preferably concentrated on the inlet side of the filterelements 15, where soot accumulates.

While FIG. 3 illustrates The oxidation catalyst 23 in a separate brickupstream of a filter 24 containing an SCR catalyst, an oxidationcatalyst and an SCR catalyst can be distributed in any suitable fashionwithin an exhaust system comprising a combined filter/SCR catalystaccording to the present invention. In one embodiment, the two catalystare co-dispersed, but generally they are dispersed separately with oneupstream of the other. The advantage of having the oxidation catalystupstream of the SCR catalyst is that it converts NO to NO₂, whichfacilitates the ammonia SCR reaction. The disadvantage of having theoxidation catalyst upstream of the SCR catalyst is that, in someconfigurations, the oxidation catalyst may oxidize ammonia.

In one embodiment, the SCR catalyst is upstream of the oxidationcatalyst. For example, the SCR catalyst can be formed in a washcoat onthe inlet side of the filter, while the oxidation catalyst is containedin an underlying coating.

In another embodiment, the oxidation catalyst is upstream of the SCRcatalyst, for example in a brick upstream from the filter as in FIG. 3,and ammonia is supplied between the oxidation catalyst and the SCRcatalyst. In a further embodiment, the system contains a NOx adsorbant,and the oxidation catalyst is upstream of the adsorbant and the filter.In a still further embodiment, the NOx adsorbant is associated with acatalyst that is effective for converting NO to NO₂ and the NOxadsorber/catalyst acts as the oxidation catalyst. A NOx adsorbercatalyst can for a separate brick upstream of the filter/SCR catalyst orbe incorporated within the filter/SCR catalyst as described more fullybelow.

An example of an intermittent regeneration process is one where thefilter/SCR catalyst 24 is heated to a temperature where soot reacts withoxygen. The process can be controlled by measuring the pressure dropacross the filter/SCR catalyst 24 and initiating the regenerationprocess based on the pressure exceeding a critical value. The filter/SCRcatalyst 24 can be heated by any suitable method. Examples of suitableheating methods may include electrical resistance heating and a fuelburner located upstream of the filter/SCR catalyst 24. Electricalresistance can involve applying a voltage directly to the filter/SCRcatalyst or to resistance wires permeating the device. Soot oxidation isexothermic. It may be possible to initiate the soot oxidation reactionin a localized area of the filter/SCR catalyst 24 and have the reactionpropagate through the rest of the device.

The ammonia supply 22 can be any suitable ammonia source. Examples ofammonia sources include reservoirs, such as reservoirs of ammonia,ammonium carbomate, or urea, and ammonia plants, such as plants thatform ammonia from H₂ and N₂ or from H₂ and NOx. N₂ can be obtained fromair and H₂ can be produced by a fuel reformer. Ammonia, whatever itssource, is optionally stored in one or more adsorption beds, such asmolecular sieve adsorption beds, and desorbed as needed.

FIG. 4 is a schematic illustration of an exemplary power generationsystem 30 employing the filter/SCR catalyst 24 in a differentconfiguration. The exemplary power generation system 30 comprises theinternal combustion engine 21, a NOx trap 31, an optional reductantsupply 32, an optional oxidation catalyst 23, and the filter/SCRcatalyst 24. The NOx trap 31 is regenerated intermittently. Regenerationgenerally comprises supplying reductant to the NOx trap 31. Thereductant can be obtained from the optional reductant supply 32,although reductant can also be obtained by running the engine 21 richfor a period of time. During regeneration, ammonia and some NOx arereleased from the NOx trap 31. The ammonia reacts to reduce NOx in thefilter/SCR catalyst 24. The filter/SCR catalyst 24 can include anammonia adsorbant to buffer the ammonia. The filter/SCR catalyst 24thereby improves NOx removal, reduces ammonia emissions, and reduces theamount of reductant required. The ammonia supply 22 can also beincorporated in the power generation system 30 to reduce a furtherportion of NOx remaining in the exhaust entering the device 10.

The NOx trap 31 comprises a NOx adsorption bed and a catalyst effectivefor reducing NOx in a reducing environment. In some cases, the catalystcontributes to the adsorbant function and is necessarily provided in theadsorbant bed. In other cases, the catalyst is optionally provided in aseparate bed downstream of the adsorption bed. The adsorption bedcomprises an effective amount of an adsorbent for NOx in an oxidizing(lean) environment. The NOx trap 31 desorbs and/or reduces NOx in areducing environment, provided that the lean NOx trap is in anappropriate temperature range.

The NOx trap 31 can be incorporated into the filter/SCR catalyst 24. Forexample, the NOx adsorbant and catalyst can be coated over the inletpassages of the combined filer/SCR catalyst 24. FIG. 6 provides aschematic illustration of a combined SCR catalyst, NOx adsorbant, andparticulate filter 50. An NOx adsorber/catalyst 52 coat the inlet sidesof filter elements 51, while an SCR catalyst 53 coats the outlet sidesof the filter elements 51. The adsorber/catalyst 52 preferably enrichesthe ratio of NO₂ to NO in the NO_(x) it does not adsorb.

The adsorption bed can comprise any suitable adsorbant material.Examples of adsorbant materials include molecular sieves, such aszeolites, alumina, silica, and activated carbon. Further examples areoxides, carbonates, and hydroxides of alkaline earth metals such as Mg,Ca, Sr, and Be or alkali metals such as K or Ce. Still further examplesinclude metal phosphates, such as phoshates of titanium and zirconium.

Molecular seives are materials having a crystalline structure thatdefines internal cavities and interconnecting pores of regular size.Zeolites are the most common example. Zeolites have crystallinestructures generally based on atoms tetrahedrally bonded to each otherwith oxygen bridges. The atoms are most commonly aluminum and silicon(giving aluminosilicates), but P, Ga, Ge, B, Be, and other atoms canalso make up the tetrahedral framework. The properties of a zeolite maybe modified by ion exchange, for example with a rare earth metal orchromium. Preferred zeolites generally include rare earth zeolites andThomsonite. Rare earth zeolites are zeolites that have been extensively(i.e., at least about 50%) or fully ion exchanged with a rare earthmetal, such as lanthanum. For NOx traps generally, a preferred adsorbantis an alkaline metal or an alkaline earth metal oxide loaded with aprecious metal.

The adsorbant is typically combined with a binder and either formed intoa self-supporting structure or applied as a coating over a substrate,which can be a particulate filter. A binder can be, for example, a clay,a silicate, or a cement. Portland cement can be used to bind molecularsieve crystals. Generally, the adsorbant is most effective when aminimum of binder is used. Preferably, the adsorbant bed contains fromabout 3 to about 20% binder, more preferably from about 3 to about 12%,most preferably from about 3 to about 8%.

Devices according to the present invention are generally adapted for usein vehicle exhaust systems. Vehicle exhaust systems create restrictionon weight, dimensions, and durability. For example, an adsorption bedfor a vehicle exhaust system must be reasonably resistant to degradationunder the vibrations encountered during vehicle operation.

When the adsorbant bed is not part of the filter, the adsorbant bed canhave any suitable structure. Examples of suitable structures may includemonoliths, packed beds, and layer screening. A packed bed is preferablyformed into a cohesive mass by sintering the particles or adhering themwith a binder. When the bed has an adsorbant function, preferably anythick walls, large particles, or thick coatings have a macro-porousstructure facilitating access to micro-pores where adsorption occurs. Amacro-porous structure can be developed by forming the walls, particles,or coatings from small particles of adsorbant sintered together or heldtogether with a binder.

Preferably an NOx adsorption bed has a large capacity for adsorbing aNOx species at a typical exhaust temperature and NOx partial pressure.Preferably, the adsorbant can adsorb at least about 3% of a NOx speciesby weight adsorbant at a typical exhaust temperature and 1 torr partialpressure of the NOx species, more preferably at least about 5% by weightadsorbant, and still more preferably at least about 7% by weightadsorbant. The weight of adsorbant does not include the weight of anybinders or inert substrates. Depending on the application, a typicalexhaust temperature may be 350° C.

The NOx adsorbant bed preferably comprises a catalyst for the reductionof NOx in a reducing environment. The catalyst can be, for example, oneor more precious metals, such as Au, Ag, and Cu, group VIII metals, suchas Pt, Pd, Ru, Ni, and Co, Cr, Mo, or K. A typical catalyst includes Ptand Rh, although it may be desirable to reduce or eliminate the Rh tofavor the production of NH₃ over N₂. Effective operating temperaturesare generally in the range from about 200 to about 450° C. Lowertemperatures may also be desirable in terms of favoring the productionof NH₃ over N₂.

The catalyst of the NOx trap 31 can also serve the function of theoptional oxidation catalyst 23: providing NO₂ for continuous oxidationof soot in the device 10. A further option is to provide the NOx trap 31with an additional catalyst for the sole purpose of oxidizing some ofthe escaping NO to NO₂. Such a catalyst is preferably concentrated nearthe outlet of the NOx trap 31.

The reductant source 32 can supply any suitable reductant. Examples ofsuitable reductants include synthesis gas (syn gas), hydrocarbons, andoxygenated hydrocarbons. Syn gas includes H₂ and CO. The reductant canbe a fuel for the internal combustion engine 21. The fuel can beinjected into the exhaust.

The reductant source 32 is preferably a fuel reformer producing simplehydrocarbons, such as syn gas. Simple hydrocarbons are generally morereactive than more complex hydrocarbons in regenerating the NOx trap 31.A fuel reformer can be a catalytic reformer, a steam reformer, anautothermal reformer, or a plasma reformer. A reformer catalyst is onethat favors the production of CO and H₂ (syn gas) and small hydrocarbonsover complete oxidation of the diesel fuel to form CO₂ and H₂O. Examplesof reformer catalysts include oxides of Al, Mg, and Ni, which aretypically combined with one or more of CaO, K₂O, and a rare earth metalsuch as Ce to increase activity. A reformer would generally be suppliedwith a fuel for the internal combustion engine 21. The reformer wouldalso be supplied with an oxygen source, such as air or lean exhaust.Lean exhaust can be drawn from a high pressure portion of the exhaustsystem, such as from a manifold upstream of a turbine used in a turbocharge system. A fuel reformer is optionally placed directly in theexhaust stream.

During regeneration, sufficient reductant must be provided to consumefree oxygen in the exhaust while leaving enough reductant left over toregenerate the NOx trap 31. The reaction of free oxygen can take placeeither before the NOx trap 31 or in the NOx trap 31. In one embodiment,the reaction with oxygen takes place in a fuel reformer provided in theexhaust stream. In another embodiment, the reductant is injected in twoparts. A first part is a fuel directly injected into the exhaust toconsumer excess oxygen. A second part is syn gas, which is lessefficient for consuming excess oxygen, but more efficient for reducingNOx.

Any suitable strategy can be used to control the regeneration of the NOxtrap 31. As opposed to a simple periodic regeneration scheme, thecontrol scheme can involve determination of one or more of the followingparameters: the time at which a regeneration cycle is initiated, theduration of a regeneration cycle, and the reductant concentration duringa regeneration cycle.

One method of determining when to initiate a regeneration cycle involvesmeasuring the NOx concentration downstream of the device 10. When thisconcentration exceeds a target level, regeneration begins.

During regeneration, some NOx desorbs from the NOx trap 31. Particularlyduring the first part of the regeneration cycle, some NOx escapes theNOx trap 31 un-reacted. If there is no stored ammonia in the device 10and no ammonia supply 22, the escaping NOx is released into theatmosphere. To avoid this, in one embodiment regeneration begins whileammonia remains in the device 10.

Regeneration can be initiated based on the concentration of ammonia inthe device 10 falling to a critical value. This approach involvesmaintaining an estimated of the amount of ammonia in the device 10.Maintaining this estimate generally involves measuring ammonia and NOxconcentrations between the NOx trap 31 and the device 10.

Another control strategy is simply focused on increasing ammoniaproduction during regeneration of the NOx trap 31. When an NOx trap issaturated with NOx, relatively little ammonia production is observed.Over the course of a regeneration cycle for a saturated NOx trap, as theamount of NOx in the trap decreases, ammonia production increases. Bystarting the regeneration cycle prior to saturation, the production ofammonia in favor of N₂ can be increased. Accordingly, in anotherembodiment, regeneration begins when the NOx trap 31 reaches a certainlevel of saturation, which is preferably in the range from about 5 toabout 50% saturation, more preferably from about 10 to about 30%saturation. The degree of saturation can be estimated from measurementsor a model-based estimate of the amount of NOx in the exhaust and amodel for the NOx trap 31's adsorption efficiency and capacity.Preferably, the control scheme is effective whereby the fraction ofadsorbed NOx converted to ammonia is at least about 20%, more preferablyat least about 40%.

Using the foregoing control method, the amount of ammonia released fromthe NOx trap 31 may exceed the amount of NOx passing through the NOxtrap 31. This excess ammonia can be used to reduce a stream of exhaustthat bypasses the NOx trap 31. The ability to produce excess ammoniaallows an NOx trap to function as the ammonia supply 22. Similarly,excess ammonia production is useful in a system with two or moreadsorbers as described more fully below.

In another embodiment, regeneration is timed to control a ratio betweentotal ammonia and NOx released by the NOx trap 31. The ratio may betargeted at one to one (a stoichiometric ratio), whereby the ammoniaproduced by the NOx trap 31 is just enough to reduce the NOx passingthrough to the device 10. Preferably, however, the ratio is slightlyless, whereby ammonia slip can be avoided. A lesser amount of ammonia ispreferably from about 60 to about 95% of a stoichiometric amount. Theamount may also be reduced by an efficiency factor accounting for thefact that, depending on the structure, catalyst loading, and temperatureof the device 10, a significant fraction of the NOx supplied to thedevice 10 may not react with ammonia even when adequate ammonia isavailable. Feedback control can be used to obtain the target ratio. Inparticular, the time between regeneration cycles can be shortened toincrease ammonia production and lengthened to decrease ammoniaproduction, with the ultimate goal of creating a balance between ammoniaproduction and NOx emission from the NOx trap 31.

A control strategy can also be used to determine when to terminate aregeneration cycle, as opposed to the alternative of terminating theregeneration cycle after a fixed or pre-determined period of time.Typically, the amount of NOx in the NOx trap 31 can be determined fromvehicle operating conditions and a few measurements. The amount ofreductant required to regenerate the NOx trap 31 can then be calculated.Nevertheless, it can be advantageous to use feedback control todetermine when to conclude a regeneration cycle. In a preferredembodiment, a regeneration cycle is terminating according tomeasurements of the ammonia concentration downstream of the NOx trap 31.

As a regeneration cycle progresses, the ammonia concentration downstreamof an NOx trap 31 first increases, then decreases. The regenerationcycle can be terminated at any recognizable point in the ammoniaconcentration curve. Most preferably, the regeneration cycle is endedupon the ammonia concentration falling below a target value following apeak. As the ammonia concentration is falling, progressively more unusedreductant is slipping through the NOx trap 31. Therefore, the targetvalue is a design choice reflecting a trade-off between maximizingammonia production and minimizing reductant slip.

Another control strategy relates to the rate at which reductant isinjected. Reductant injection rate can be targeted to a particularequivalence ratio. An equivalence ratio is based on the fuel-air mixtureas supplied to the engine 21, with a stoichiometric ratio having anequivalence ratio of one. Additional reductant injected into the exhaustdownstream of the engine 21 is figured into the equivalence ratio justas if it were supplied to the engine 21.

In one embodiment, the reductant injection rate is maximized subject toa limit on reductant breakthrough. Generally, increasing the equivalenceratio increases the ammonia production rate and minimizes theregeneration time. Where the reductant is injected into the exhaust,reducing the regeneration time reduces the fuel penalty. Duringregeneration, reductant must be supplied to consume free oxygen in theexhaust. This reductant is in excess of the reductant used to reduceNOx. The total amount of oxygen to consume depends on the length of theregeneration cycle. If the regeneration cycle is shorter, the molar flowof oxygen that must be reduced is less.

In a preferred embodiment, the reductant breakthrough rate is determinedby an oxidizable species sensor downstream of the device 10. Alloxidizable species can be considered reductant. For purposes of control,the breakthrough rate is preferably expressed as a fraction of theinjection rate in excess of the injection rate required to consume freeoxygen. For example, if doubling the excess injection rate over theamount required to consume free oxygen only doubles the breakthroughrate, the fractional conversion of reductant has not decreased at all.In one embodiment, the reductant injection rate is controlled to givefrom about 50 to about 95% conversion of reductant in excess of theamount required to consume free oxygen, in another embodiment from about70 to about 90% conversion.

Another method of reducing the fuel penalty is to employ a dual adsorbersystem as schematically illustrated by the exhaust system 40 of FIG. 5.The exhaust system 40 has twin lean NOx traps 31A and 31B, thefilter/SCR catalyst 24, and an optional clean-up oxidation catalyst 41all contained in a single housing 42. The exhaust flow can be divertedto one or the other NOx traps by a damper 43. Injection ports 44A and44B are configured to provide reductant to one or the other of the NOxtraps 31A and 31B. Sample ports 45A and 45B are provided to sample theoutflows of the NOx traps 31A and 31B respectively for purposes ofcontrol. Rather than use sample ports, sensors can be placed inside thehousing 42. The outflows of NOx traps 31A and 31B combine after passingthrough baffling device 46, which is designed to promote mixing of thetwo streams. After passing through the filter/SCR catalyst 24, theexhaust is treated by the oxidation catalyst 41 to convert escapingammonia and reductant to more benign species.

One advantage of a dual adsorber system is that reducing agent does notneed to be wasted consuming free oxygen in the exhaust duringregeneration. Another advantage is that the reducing agent does not needto be diluted with the exhaust. This increases the concentration of thereducing agent and thereby the efficiency with which it reacts. Afurther advantage is that the residence time of the reducing agent inthe NOx traps 31A and 31B can be increased. The residence time can beincreased both because the residence time is not limited by the exhaustflow rate and because more time can be taken to regenerates the NOxtraps. A longer residence time allows for a higher conversion efficiencyfor a given amount of catalyst.

Additional advantages can be realized when the outflows of the NOx trapsare combined. One advantage is that excess reductant from the NOx trapsand ammonia slipping from the filter/SCR catalyst 24 can be reduced bythe oxidation catalyst 41 without injecting oxygen. In a system thatdoes not have a unified flow, there is no free oxygen in the exhaustdownstream of the NOx traps during regeneration. Air must be injected oranother oxygen source provided to oxidize unconverted hydrocarbons andNH₃. With a unified flow, ample oxygen is generally supplied by theexhaust.

Another advantage of a unified flow is that the ammonia production ratefrom one of the NOx traps 31A and 31B can be controlled to match the NOxflow rate from the other of the traps, whereby the NOx and NH₃ ratesinto the filter/SCR catalyst 24 remain in an approximately fixedproportion. Total ammonia production can be controlled through thefrequency of regeneration and the reductant concentration and the rateof ammonia production can be controlled through the rate at whichreductant is supplied.

To allow a unified flow, the pressure of the reductant injection must beregulated to a level above that of the exhaust at the point where thestreams join. This can be accomplished without extra pumps, even whenthe reductant is syn gas. For example, syn gas can be generated fromexhaust drawn from a high pressure point in the exhaust system and fueldrawn from a common rail. The feeds can be reacted while remaining at anelevated pressure.

According to another concept of the invention, a particulate filter alsoacts as a NOx adsorber. The filter elements of the device are made withthe adsorbant material. FIG. 7 provides a schematic illustration of anexemplary wall-flow particulate filter/NOx adsorber 70 in which thefilter elements 71 comprise the adsorbant material. Preferably, thefilter elements 71 comprise at least about 40% by weight adsorbant, morepreferably at least about 60% by weight adsorbant, and still morepreferably at least about 80% by weight adsorbant. In a preferredembodiment, the filter elements 71 are made up of adsorbant-containingparticles bound together by sintering or held together with a binder.The filter pores are spaces between the particles. Theadsorbant-containing particles together with any binder can be extrudedto form a monolith structure of a wall flow filter. Alternating passagescan be plugged at either end to direct the flow through the filterwalls. The porosity of the filter can be controlled through the particlesizes. For example, particle sizes in the range from about 3 to about 20μm may be appropriate. The particles themselves may have micro-porosityto allow effective utilization of the entire adsorbant mass.

The combined adsorbant/diesel particulate filter can be used inconjunction with an ammonia SCR catalyst. The catalyst can be placeddownstream of the device or incorporated into the device. The catalystcan be incorporated into the device by coating the internal surfaces ofthe filter elements with an SCR catalyst-containing wash coated.Alternatively a highly porous wash coat can be used that lies on top ofthe filter elements. Where the later type of wash coat is used, it canbe selectively applied to the outlet side of the device. FIG. 8 providesan example 80, in which an SCR catalyst is provided in a wash coat 72 onthe outlet side of the filter elements 71.

Another way of integrating the SCR catalyst is to co-disperse it withthe adsorbant. For example, fine particles of an NOx-adsorbant, such asa NOx-adsorbing zeolite impregnated with a NOx trap catalyst, can becombined with fine particles of an SCR catalyst, for example an ammoniaSCR catalyst zeolite or an ammonia-adsorbing zeolite impregnated with anammonia SCR catalyst. The mixed particles can be combined in a wash coatover a supporting structure or formed into a self-supporting structureby sintering or binding.

Ammonia-adsorbing zeolites include faujasites and rare earth zeolites.Faujasites include X and Y-type zeolites. Rare earth zeolites arezeolites that have been extensively (i.e., at least about 50%) or fullyion exchanged with a rare earth metal, such as lanthanum.

The invention as delineated by the following claims has been shownand/or described in terms of certain concepts, aspects, embodiments, andexamples. While a particular feature of the invention may have beendisclosed with respect to only one of several concepts, aspects,examples, or embodiments, the feature may be combined with one or moreother concepts aspects, examples, or embodiments where such combinationwould be recognized as advantageous by one of ordinary skill in the art.Also, this one specification may describe more than one invention andthe following claims do not necessarily encompass every concept, aspect,embodiment, or example contained herein.

1. A particulate filter, comprising: a body having an entrance and anexit; and filter elements adapted to filter particulate matter from gasflowing between the entrance and the exit; wherein the filter elementscomprise at least about 40% by weight of an NOx adsorbant.
 2. Theparticulate filter of claim 1, wherein the filter elements comprise atleast about 60% by weight NOx adsorbant.
 3. The particulate filter ofclaim 1, wherein the filter elements further comprise a catalyst for thereduction of NOx.
 4. The particulate filter of claim 3, wherein thecatalyst for the reduction of NOx is also effective for oxidizing NO toNO₂.
 5. The particulate filter of claim 1, wherein the filter elementsare formed by combining NOx adsorbant particles into a cohesive mass. 6.The particulate filter of claim 1, wherein the filter elements compriselarge pores formed by interstices between NOx adsorbant particles. 7.The particulate filter of claim 1, further comprising an effectiveamount of an SCR catalyst.
 8. The particulate filter of claim 7, whereinthe SCR catalyst is contained in a wash coat on an exit side of thefilter elements.
 9. A power generation system comprising the particulatefilter of claim
 1. 10. The power generation system of claim 9, furthercomprising a fuel reformer configured to supply reformed fuel forregenerating the NOx adsorbant.
 11. A vehicle comprising the powergeneration system of claim
 9. 12. A method of cleaning a diesel-poweredvehicle's exhaust, comprising: passing the exhaust through a particulatefilter comprising at least about 40% by weight NOx adsorbant tosubstantially reduce both a particulate matter and an NOx content of theexhaust; and intermittently regenerating the NOx adsorbant by creating areducing atmosphere within the particulate filter; wherein theparticulate filter is mounted on the vehicle.
 13. The method of claim12, wherein the particulate filter comprises filter elements shaped frommaterial comprising the NOx adsorbant.
 14. The method of claim 12,wherein regenerating the NOx adsorbant comprises supplying syn gas tothe particulate filter.
 15. The method of claim 12, wherein theparticulate filter comprising at least about 60% by weight of the NOxadsorbant.
 16. The method of claim 12, further comprising passing theexhaust over an effective amount of an ammonia SCR catalyst.
 17. Themethod of claim 16, wherein the ammonia SCR catalyst is supported by theparticulate filter.
 18. The method of claim 17, wherein the ammonia SCRcatalyst comprises an effective amount of catalyst selected from thegroup consisting of TiO₂, WO₃, V₂O₅, and MoO₃ and combinations thereof.19. The method of claim 12, further comprising oxidizing NO to NO₂within the filter.
 20. The method of claim 16, further comprisingoxidizing NO to NO₂ within the filter.